1. Field of the Invention
The present invention relates to a device to compensate for the non-linearity of an analog/digital converter. It can be applied to the making of digital communications receivers.
A major characteristic of radio receivers is their capacity to demodulate weak signals in the presence of high-amplitude disturbing signals. This capacity is qualified by an instantaneous dynamic range and is measured in decibels.
2. Description of the Prior Art
In analog receivers, the instantaneous dynamic range is obtained by narrow-band filtering upline from the amplifier system, and may reach up to 120 dB.
By principle, wide-band digital receivers do not have any narrow frequency band filter placed before the analog/digital filter. The effect of this is to greatly reduce the dynamic range of the receivers and cause distortions in the demodulated signal.
The dynamic range of the analog/digital converters used is about 70 dB. Their spurious-free dynamic range or S.F.D.R. is 80 dB in relation to the full scale or FS of the converter. This dynamic range varies as a function of the level of the signal applied to the converter, for example for signals at -20 dB of the full scale the dynamic range is limited to 20 dB.
The distortions of the demodulated signal are produced by lack of precision in conversion since the steps of a converter do not all have the same analog value. The differences from one step to another may indeed attain plus or minus 50%. This non-linearity, which is known in the literature as "differential linearity" is said to be coarse as opposed to the non-linearity of analog circuits which is smooth. For an analog/digital converter, there is actually a multitude of curves with differential linearities of the type shown in FIG. 1. This figure shows an intermodulation distortion curve in two tones that are equal as a function of the level of the signal applied to the input of an analog/digital converter. These two tones depend both on the absolute frequency of the signal and its ratio with the clock signal.
A known method of compensating for the non-linearity of a digital-analog converter consists of the adding, to the signal applied to the input of the converter, of a noise signal called dither, having special characteristics. The efficiency of the method is not optimal for correcting the non-linearity of analog/digital converters of digital radio receivers with wide frequency bands extending over several tens of MHz and low modulation frequency bands of some tens of kHz. Thus for example, by adding a dither signal at -35 dB/FS in terms of mean power, the distortion curve of FIG. 1 is improved as shown in FIG. 2 for peak levels of -10 dB/FS to -25 dB/FS. This improvement is even more marked for the lower levels. It can also be seen that the roughness has diminished. However, the third-order intermodulation (IMD3) remains always below -80 dBc.
An even closer examination of the curve of FIG. 2 shows that, to obtain the benefit of maximum linearity, it would be appropriate to position the interference signal at the full scale of the converter. This would mean increasing the gain by about 20 dB in the analog part of the receiver located before the converter, although this is not justified by the noise floor of the converter and could lead to instability in the receiver through the increase resulting therefrom in the dynamic range of the automatic gain control (AGC) analog loop placed before the converter. However, because of the stepped management of the AGC loop and because of the hysteresis needed, it is practically impossible to position the signal at a value outside the upper limit of window of -3 dB to -8 dB of the full scale. This means a corresponding loss in the instantaneous dynamic range. Finally, the fact of positioning the natural interference signals at the maximum of the full scale leaves no margin whatsoever, and the pulsed noises are clipped by the converter with the resulting inconvenience.